Rechargeable alkali metal polymer batteries manufactured from laminates of solid polymer electrolytes and sheet-like anodes and cathodes display many advantages over conventional liquid electrolyte batteries. These advantages include having a lower overall battery weight, having a high power density, having a high specific energy and having a longer service life, as well as being environmentally friendly since the danger of spilling toxic liquid into the environment is eliminated.
The components of solid polymer electrochemical cells include positive electrodes, negative electrodes and separators capable of permitting ionic conductivity, such as solid polymer electrolytes, sandwiched between each anode and cathode pair. The negative electrodes, or anodes, and the positive electrodes, or cathodes, are made of material capable of reversibly releasing and occluding alkali metal ions.
The cathodes are typically formed of a mixture of active material capable of occluding and releasing lithium, such as transitional metal oxides or transitional metal phosphates, an electronically conductive filler, usually carbon or graphite or combinations thereof, and an ionically conductive polymer binder. Cathode materials are usually paste-like materials that require a current collector, usually a thin sheet of electrically conductive material, such as aluminum foil.
The anodes are typically made of light-weight metal foils, such as alkali metals and alloys. Typically, anodes are made of lithium metal, lithium oxide, lithium-aluminum alloys and the like. Alternatively, the anodes may be made of composite paste-like material, such as carbon-based intercalation compounds in a polymer binder, in which case the anodes also require a current collector support, for example a thin sheet of copper.
During discharge, the electrochemical reaction involves the oxidation of the lithium metal anode and the reduction of the transitional metal oxide cathode. During discharge, the lithium cations, Li+, travel through the ionically conductive polymer separator and are inserted into the interstitial sites of the transitional metal oxide cathode, while the electrons provided by anode oxidation generate electrical current. When recharging the lithium electrochemical cells, electrical current is provided to the anode with the effect of removing the lithium cations, Li+, from the interstitial sites of the transitional metal oxide cathode, returning them to the lithium anode. In theory, the electrochemical reaction is completely reversible; however, in practice, it may not be possible to restore the electrochemical cells to their original state through a normal charge, because the voltage limits of the application load to which the electrochemical cells are connected may prevent a full charge. When the electrochemical cells are not fully recharged or restored, some of the inserted lithium cations remain within the interstitial sites of the transitional metal oxide cathode, causing an excessive number of charge/discharge cycles. As such, the capacity of each electrochemical cell may be prematurely reduced by the remaining lithium cations within the transitional metal oxide cathode. Because of the voltage limit of the application load, the electrochemical battery may suffer an artificially accelerated capacity fade, which may reduce its useful life.
Thus, there exists a need for a method and process of charging an alkali metal electrochemical generator, adapted to circumvent voltage limits imposed by application loads to which the generator is connected, such that each electrochemical cell of the electrochemical generator may be restored to its original chemical state.